GROWTH HISTORY OF STROMATOLITES IN A HOLOCENE FRINGING-REEF, STOCKING-ISLAND, BAHAMAS

A stromatolite and algal ridge reef complex 2.1 m thick fringes the ea st coast of Stocking Island, Exuma Gays, Bahamas, This reef was establ ished on a Pleistocene calcarenite terrace about 4500 yr BP. Stromatol ites, which occur in back-reef and reef-flat zones, are up to 1 m thic k and were constructed by cyanobacterial-dominated communities, Study of the growth history of these stromatolites, ranging in scope from fa cies analyses to details of microfabric construction, presents new per spectives on stromatolite formation. Lithologies identified in eight c ores from across the Stocking Island reef complex, together with plots of 13 radiocarbon dates in relation to a Bahamian sea-level curve, in dicate that this reef began as an intertidal vermetid gastropod buildu p, Subsequent flooding of the Pleistocene terrace allowed the branchin g coralline alga Neogoniolithon strictum to overgrow the vermetids and eventually form an emergent algal ridge about 1500 years ago. Shiftin g sands accumulated in the lee of this ridge and excluded most benthic communities and herbivores, thereby promoting growth of cyanobacteria l mats that formed stromatolite buildups. With a decrease in wave ener gy over the last 500 years, possibly due to the growth of offshore pat ch reefs, the urchin Echinometra lucunter colonized the algal ridge. R esultant bioerosion by this urchin destroyed the emergent part of the ridge and is now undercutting the seaward edge of the stromatolite bui ldups. Lamination in the Stocking Island stromatolites results from ea rly lithification processes in cyanobacterial mats, possibly in respon se to biogeochemical changes in the mats during hiatuses in sediment a ccretion. These processes, which create partially indurated laminae wi th a distinct microstructure, involve precipitation of thin micrite cr usts, intense microboring along a surface below this crust, micritizat ion of sediment grains, and precipitation of point-contact cement betw een micritized grains, Introduction of turf algae to the cyanobacteria l mat community disrupts formation of the lithified laminae, thereby i nhibiting stromatolite development.